drm/fb: fix CONFIG_DRM=m && CONFIG_FB=n

The previous commit to move the parsing into the core drm created a
new situation and a soft dependency on the CONFIG_FB. We really don't
want to make this a hard dependency so just wrap the one place that
actually needs an fb symbol.
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_modes.c

@@ -1000,8 +1000,10 @@ bool drm_mode_parse_command_line_for_connector(const char *mode_option,
 	int i;
 	enum drm_connector_force force = DRM_FORCE_UNSPECIFIED;
 
+#ifdef CONFIG_FB
 	if (!mode_option)
 		mode_option = fb_mode_option;
+#endif
 
 	if (!mode_option) {
 		mode->specified = false;